If you know about ZFS snapshots, you know that one of their famous properties is that they're immutable; once a snapshot is made, its state is frozen. Or so you might casually describe it, but that description is misleading. What is frozen in a ZFS snapshot is the state of the filesystem (or zvol) that it captures, and only that. In particular, the metadata associated with the snapshot can and will change over time.

(When I say it this way it sounds obvious, but for a long time my intuition about how ZFS operated was misled by me thinking that all aspects of a snapshot had to be immutable once made and trying to figure out how ZFS worked around that.)

One visible place where ZFS updates the metadata of a snapshot is to maintain information about how much unique space the snapshot is using . Another is that when a ZFS snapshot is deleted, other ZFS snapshots may require updates to adjust the list of snapshots (every snapshot points to the previous one) and the ZFS deadlist of blocks that are waiting to be freed.

Mechanically, I believe that various things in a dsl_dataset_phys_t are mutable, with the exception of things like the creation time and the creation txg, and also the block pointer , which points to the actual filesystem data of the snapshot. Things like the previous snapshot information have to be mutable (you might delete the previous snapshot), and things like the deadlist and the unique bytes are mutated as part of operations like snapshot deletion. The other things I'm not sure of.

(See also my old entry on a broad overview of how ZFS is structured on disk . A snapshot is a 'DSL dataset' and it points to the object set for that snapshot. The root directory of a filesystem DSL dataset, snapshot or otherwise, is at a fixed number in the object set; it's always object 1. A snapshot freezes the object set as of that point in time.)

PS: Another mutable thing about snapshots is their name, since ' zfs rename ' can change that. The manual page even gives an example of using (recursive) snapshot renaming to keep a rolling series of daily snapshots.